The present invention relates generally to a catadioptric imaging microscope system, and more specifically to an imaging system that is capable of reimaging its system pupil to a location outside of the catadioptric objective lens section.
Generally, the industry of semiconductor manufacturing involves highly complex techniques for integrating circuits into semiconductor materials. Due to the large number of processing steps and the decreasing size of semiconductor devices, the semiconductor manufacturing process is prone to processing defects that decrease device yields. Testing procedures to eliminate these processing defects from the processing steps are therefore critical for maintaining high yielding production facilities. Since the testing procedures are an integral and significant part of the manufacturing process, the semiconductor industry constantly seeks more sensitive and efficient testing procedures.
One type of testing system uses optical imaging technology to create an image of a semiconductor wafer or die. The image is then inspected to detect defects on the semiconductor wafer or die. One type of imaging system, a catadioptric imaging system for the deep ultraviolet spectral region (about 0.23 to 0.37 micron wavelength) is disclosed by U.S. Pat. No. 5,031,976 to Shafer and U.S. Pat. No. 5,488,229 to Elliot and Shafer. These systems employ the Schupmann achromatic lens principle and the Offner-type field lens. A catadioptric imaging system with added wide-range zoom capabilities is disclosed in U.S. Pat. No. 5,999,310 to Shafer et al. In addition to the zooming capability, the system disclosed in U.S. Pat. No. 5,999,310 has a high numerical aperture and performs in the ultra-broad band spectral region.
Even though the catadioptric imaging systems provide images having excellent quality, the systems are limited with respect to the number of testing modes in which they may operate. This results from the fact that the system pupil is located within the catadioptric lens section of the imaging system. The ability to access the system pupil with enough space around it to allow for the insertion of a beamsplitter, so that an illumination light path may be separated from a collection light path, would give the system the ability to operate under additional optical inspection modes. The imaging system is configured to operate under these additional inspection modes by selectively controlling the amount and/or the pattern of light travelling along these light paths. These modes, in turn, allow the imaging system to operate with greater detection sensitivity. However, since the system pupil in the current imaging systems is located within the catadioptric lens section, it is difficult to access, thereby making the systems difficult to configure. Attempts to use the buried pupil for different imaging modes causes separation between the illumination aperture and the collection aperture. This results in a magnification and position difference between them. This means that any attempt to economically position light between the apertures results in light loss because the apertures must be sized to minimize interference between the optical signals (crosstalk).
Another drawback of the current systems is that they are not capable of operating in a telecentric mode for either ring darkfield or full-sky illumination/collection modes. Being non-telecentric means that the imaged appearance of a wafer structure at the center of the field of view will be different from the appearance of the same structure at the edge of the field of view. This is due to the difference between the angle in which light is incident upon the structures and the angle from which the structures are viewed.
Yet another drawback to the current systems lies in the fact that the pupil plane is aberrated. These aberrations cause two unwanted effects at the pupil. The first effect is that of an increased spot size compared to an unaberrated spot, and the second is an error in the location of the spot relative to where it should be located (distortion). The availability of a high-quality pupil plane allows for Fourier filtering, which is effective in increasing defect detection sensitivity for certain periodic wafer structures. Fourier filtering will not be possible unless the Fourier plane has sufficient quality and is physically accessible to allow the insertion of a mechanical Fourier filter.
In view of the of the foregoing, an optical imaging system having a pupil which is physically accessible, which is able to operate in a telecentric mode, is configurable to operate in various optical testing modes to obtain high degrees of defect detection sensitivity, and allows for more accurate Fourier filter testing would be desirable.
The present invention is directed to a broad spectral region catadioptric optical system that allows for easy access to its pupil, is able to operate in a xe2x80x9ctelecentric-in-object-spacexe2x80x9d mode for ring darkfield and full-sky illumination modes, and allows for more accurate Fourier filtering. One aspect of the invention pertains to an optical system which includes three lens sections, a catadioptric objective lens section, a reimaging lens section and a zoom lens section, which are all aligned along the optical path of the optical system. The reimaging lens section re-images the system pupil such that the re-imaged pupil is accessible separately from any of the lens sections. In one embodiment, the reimaging lens section includes an intermediate focus lens group, which is used to create an intermediate focus, a recollimating lens group, which is used to recollimate the light traveling from the intermediate focus lens group, and a refocusing lens group, which generates the re-image of the pupil. The optical system may also include a beamsplitter, which creates a separated illumination pupil and collection pupil. The illumination pupil and the collection pupil may then be manipulated with an illumination aperture and a collection aperture, respectively, so that the optical system may operate in various test modes such as brightfield, ring darkfield and full-sky illumination. The reimaging lens section also allows the system to be configured to operate in laser darkfield mode.
Another aspect of the invention pertains to a method for using the broad spectral region catadioptric optical system. The method includes the operations of directing radiation from a radiation source so that the radiation passes through the illumination pupil and the collection pupil of the optical system. The method also includes operations for configuring the illumination pupil and the collection pupil so that the optical system may operate in brightfield, ring darkfield and full-sky illumination modes.
These and other features and advantages of the present invention will be presented in more detail in the following specification of the invention and the accompanying figures, which illustrate by way of example the principles of the invention.